420 UTILIZATION OF MINERAL SALTS 



vestigators to be convincing enough to warrant their addition to the list of 

 certainly necessary elements. Future investigations may eventually result in 

 further additions to the list of essential elements. 



Specific Roles of the Mineral Elements in Plants. — Although it is 

 conventional to speak of the "ash" and "non-ash" elements in plants, from 

 the standpoint of plant metabolism this is a purely artificial distinction. Of 

 the non-ash elements the roles of carbon, hydrogen, and oxygen in the syn- 

 thesis of carbohydrates, fats, and related compounds have already been dis- 

 cussed. There are few organic compounds of physiological importance in 

 either plants or animals which do not contain all three of these elements. 

 The role of nitrogen, the other non-ash element, in the synthesis of proteins 

 and other important nitrogen-containing compounds will be considered in the 

 following chapter. Each of the essential mineral elements is known to play 

 certain specific roles in plants not to be entirely described in terms of the gen- 

 eral functions of mineral elements which have already been summarized. 



The parts played by the mineral elements in plant metabolism are incom- 

 pletely understood and it is probable that each of them is involved in meta- 

 bolic and physico-chemical processes occurring in plant cells in ways which 

 are not at present recognized. When an element is a constituent of some 

 important plant compound — such as the sulfur in proteins, or the magnesium 

 in chlorophyll — that particular role of the element is relatively easy to identify. 

 When, however, an element is a necessary link in one of the many complex 

 chain reactions occurring in plant cells, its exact role is usually obscure and 

 is often difficult to recognize. 



Sulfur. — As a rule this element seems to be fairly well distributed through- 

 out the organs and tissues of the plant. Sulfur is a constituent of the amino 

 acid cystine which in turn is one of the compounds from which plant proteins 

 are made (Chap. XXVI). It is also a constituent of glutathione j a com- 

 pound which is supposed by many investigators to play a fundamental part 

 in respiration processes (Chap. XXX) and of the mustard oil glycosides such 

 as sinigrinj which impart characteristic odors and flavors to such species as 

 mustards, onions, and garlic. Much of the sulfur present in plants often 

 remains in the inorganic form as sulfates. Most of the sulfur in the organic 

 molecules present in plants occurs in reduced forms. Sulfur which has ac- 

 cumulated in one organ of a plant may subsequently be redistributed to other 

 organs. 



The presence of an abundance of sulfur in the soil apparently favors 

 root formation in many species. Chlorophyll development is often retarded 

 in sulfur deficient plants. The pale green color of such plants soon changes 

 to a deep green when sulfur is applied. An abundance of sulfur also favors 



